Digital printing involves technologies in which a printed image is created directly from digital data, for example using electronic layout and/or desktop publishing programs. Known methods of digital printing include full-color ink-jet printing, electrophotographic printing, laser photo printing, and thermal transfer printing methods.
Electrophotographic printing techniques involve the formation of a latent image on a photoconductor surface mounted on an imaging plate or other surface. In one instance, the photoconductor is first sensitized, usually by charging, and then exposed to light projected through a positive film of the document to be reproduced, resulting in dissipation of the charge in the areas exposed to light. The latent image is subsequently developed into a full image by the attraction of oppositely charged toner particles to the charge remaining on the unexposed areas. For liquid electrophotographic inks, the developed image is transferred from the photoconductor to a hot elastomeric blanket, from which it is transferred to a substrate, such as paper, plastic or other suitable material, by heat or pressure or a combination of both to produce the printed final image.
The latent image can be developed using, either a dry toner (a colorant mixed with a powder carrier) or a liquid ink (a suspension of a pigmented resin in a liquid carrier). The toner or ink generally adheres to the substrate surface with little penetration into the substrate. The quality of the final image is largely related to the size of the particles, with higher resolution provided by smaller particles.
Liquid inks used in liquid electrophotography are generally composed of pigmented thermoplastic resin particles suspended in a non-conducting liquid carrier, generally a saturated hydrocarbon. Offset-preventing and release-facilitating oil, such as silicone oil, is often used to increase the efficiency of ink transfer from the imaging surface. The liquid ink is electrostatically charged and brought into contact with the photoconductor surface to develop the latent image. When transferred to a hot elastomeric blanket and heated, the particles melt and fuse to form a tacky polymer film. When the tacky polymer film comes in contact with a cooler surface, such as a paper substrate, the film hardens and adheres to the substrate and peels away from the blanket, laminating the paper. The ink is deposited onto the substrate essentially dry, and desired print finishing can be performed immediately. Since the ink is transferred completely from the blanket to the substrate, a new layer in a different color can be created for every rotation of the press.
The small particle size used in liquid inks allows the printing of high resolution, high gloss images with sharp edges and very thin image layers. However, due to the fact that the print produced by liquid ink is not absorbed into the paper but is attached to the paper surface by adhesive attraction, the print may be prone to damage by scratching, peeling, or flaking. As such, improved electrophotographic inks continue to be sought through ongoing developmental and research efforts.